The present invention relates to optical communication systems, and more particularly, to optical communication systems that are characterized by the propagation over a single optical fiber of bidirectional signals having different information capacity requirements.
The interconnection of two stations by a single optical fiber is desirable from the standpoint of cost and system simplicity. Such asymmetrical bidirectional signal propagation is employed in certain classes of optical systems including certain types of cable TV systems, data links, weapons systems and the like. In these systems the transmitted information requirements in the two directions of propagation are often dissimilar. The central station usually has the higher bandwidth requirements, but in some applications such as television security systems, the end station has higher bandwidth requirements. The lower speed path is frequently much lower in capacity than the high speed path. If information requirements are static, then many prior art bidirectional systems are available. However, such current systems fail to function if there are new requirements in the lower speed link caused by factors such as increasing user requirements or a need to improve the reliability or responsiveness of the link.
When optical signals are propagated bidirectionally over a single fiber, the transmitters at opposite ends of the fiber often differ in some respect such as wavelength or modulation format in order to avoid crosstalk between the transmitted and received signals. When silica-based optical fibers are employed, optical signals can be propagated at wavelengths in the low attenuation windows at 700-950 nm and 1.1-1.7 .mu.m. Such systems are advantageous in that signals carried in light streams of 1300 nm or greater cannot be detected by devices used to receive light of shorter wavelengths. This reduces system cross talk and noise. Another design consideration for systems employing such fibers is the cost of the transmitter. Gallium arsenide devices, which operate in the shorter of the aforementioned wavelength regions, are less expensive than indium phosphide devices, which operate in the longer wavelength region.
A further design consideration for dual wavelength bidirectional systems is the cost, complexity and effectiveness of the wavelength division multiplexer (WDM) which connects the optical source and detector to each end of the optical fiber. In the single-mode duplex optical data transmission system disclosed in U.S. Pat. No. 4,557,553 (McLandrich) the wavelengths of the two sources are 800 nm and 850 nm. Since both transmitters operate in the 750-900 nm region, both can employ gallium arsenide, aluminum-gallium-arsenide sources. However, because of the small wavelength separation between the two sources, the system of the McLandrich patent must utilize relatively expensive WDM devices that employ evanescent field coupling to separate the two signals. Also, the McLandrich system cannot realize the aforementioned advantage of employing a detector that is sensitive to only one of the propagated wavelengths. Furthermore, it is desirable to operate the high capacity direction at a wavelength longer than the 700-950 nm window to which the McLandrich system is limited in order to obtain a better balance of the dynamic range which, inter alia, allows longer spans between repeaters.
Inexpensive wavelength division multiplexers can be employed if the wavelengths of the two transmitters are sufficiently separated, e.g. when the two transmitted wavelengths are in the two aforementioned low attenuation windows. However, the operation of conventional systems in such a bidirectional manner would cause certain problems and disadvantages. A fiber of the type taught in U.S. Pat. No. 4,715,679 is capable of providing low dispersion (less than 5 ps/km-nm) at wavelengths longer than about 1200 nm. If both transmitters operated in this wavelength range, both sources would be expensive. If one chooses an optical fiber that is designed to propagate a low dispersion single-mode signal at about 800 nm, to take advantage of the sources available at that wavelength, dispersion is very high at 1300 nm. Whereas dispersion may be a little lower at 1550 nm, the loss would be very high for that fiber at that wavelength due to the very low cutoff wavelength needed.
The source-to-fiber coupling efficiency can be enhanced in the systems under discussion by employing multimode fibers. However, the bandwidth of a conventional multimode fiber is relatively low since the group delays of modes are different. Such a system could not be upgraded because of the relatively low bandwidth of the multimode fiber. Conventional, commercially available silica-based single-mode optical fibers that are designed for operation at wavelengths beyond 1250 nm are capable of propagating two or more modes in the wavelength region between about 800 nm and 900 nm. However, such fibers exhibit a bandwidth less than 1 GHz-km (usually about 200-400 MHz-km) when operated in that few mode region.
Higher multimode bandwidth can be achieved by designing a fiber such that only a few modes propagate and such that the normalized delay times of the propagating modes coincide at or near the operating V-value V.sub.o. Also, the difference between the normalized delay times of the propagating modes caused by V-value deviation from V.sub.o should be as small as possible. For example, U.S. Pat. No. 4,204,745 (Sakai et al) discloses a +graded index two-mode fiber wherein the refractive index profile is given by ##EQU1## where n.sub.1 denotes the peak refractive index of the core, n.sub.0 denotes the refractive index of the cladding, .DELTA. is the relative index difference (n.sub.1.sup.2 -n.sub.0.sup.2)/2n.sub.1.sup.2, a is the core radius, and .alpha. is the index profile parameter. The fiber is designed such that the normalized frequency V is as large as possible, so long as V is less than the cut-off frequency of the second higher order mode LP.sub.21). Thus, the fundamental mode LP.sub.01) and the first higher order mode (LP.sub.11) are propagated simultaneously. The refractive index distribution of the core is designed such that the group delay of the lowest order LP.sub.01) mode coincides with that of the first higher order (LP.sub.11) mode, the power exponent .alpha. preferably satisfying the inequality EQU 3.2.ltoreq..alpha..ltoreq.6
Although fibers of the type disclosed in the Sakai et al. patent are capable of propagating two modes with low dispersion over a band of wavelengths, the low dispersion wavelength band is relatively narrow. Therefore, light sources having outputs within a narrow wavelength range must be employed. Since the spectral output of light sources varies with respect to temperature, the system is more sensitive to temperature change. Temperature effects can be compensated through active device thermal stabilization. However, this raises the cost of the light source module.
Fibers having .alpha.-values less than 3.2 have been proposed in the publication, K. Kitayama et al., IEEE Journal of Quantum Electronics, vol. QE-17, No. 6, June 1981, pp 1057-1063 and in the publication, L. G. Cohen et al., Bell System Technical Journal, vol. 59, No. 6, July-August 1980, pp. 1061-1072. Although the wavelength band of good mode equalization can be broadened by reducing .alpha. to a value below 3.2, this cannot be done without adversely affecting single-mode operation. When the .alpha.-value of a fiber is in the range defined by the Kitayama et al. and Cohen et al. publications, operation is substantially above the theoretical cutoff of the second higher order mode. If the second higher order mode is not totally eliminated, the system bandwidth will be degraded. This is especially troublesome in subscriber loop applications wherein relatively short lengths of fiber may connect the transmitter and receiver and wherein short lengths of fiber may extend between connectors. The second higher order mode can be re-excited at each connector. Also, if such a fiber is optimized for two mode operation, it will not exhibit low waveguide dispersion in the single-mode region.
The publication, M. M. Cvijetic et al., IEEE Journal of Quantum Electronics, vol. QE-23, No. 5, May 1987, pp. 469-472 describes a non .alpha.-profile fiber design for obtaining dispersion-free two-mode operation at 1.55 .mu.m. Two-mode operation at 1.3 .mu.m is described in the publication, M. M. Cvijetic, Optical and Quantum Electronics, vol. 16, 1984, pp 307-317. In accordance with the technique described in these publications, an attempt is made to minimize chromatic dispersion in the two-mode region of operation (either 1.55 .mu.m or 1.3 .mu.m). There is no attempt made in these publications to optimize dispersion properties of the fundamental mode in the single-mode region of operation. If an attempt were made to operate either of the two fibers of the Cvijetic publications in the single-mode region, waveguide dispersion would be so high that information carrying capacity of the fiber would be limited.
None of these prior art few mode fibers is capable of low dispersion operation in the single-mode region near the cutoff V-value of the first higher mode.
The single-mode/multimode fiber taught in U.S. Pat. No. 4,465,334 (Siemsen et al.) comprises an inner, single-mode core having a stepped refractive index profile, and its cladding is the multimode light conducting zone. The diameter of the outer zone is much larger than that of the inner zone, whereby it propagates a multimode signal. An attempt to excite only the single-mode signal by confining the source light to the central zone would result in the propagation of a significant percentage of the input power in the outer, multimode part of the fiber. Similarly, an attempt to initiate the propagation of only a multimode signal would also result in the transmission of a single-mode signal in the central zone. In either case, bandwidth would be adversely affected due to the different propagation speeds of the single-mode and multimode signals.